Rotherham were relegated from the Sky Bet Championship after promotion-chasing West Brom came from behind to secure a 2-1 victory at The Hawthorns.
The Millers looked set to keep their survival hopes alive when Clark Robertson gave them a 50th-minute lead.
But Jay Rodriguez drew Albion level from the penalty spot in the 77th minute before Rekeem Harper's first goal for the club two minutes later - on the day he was named as their Young Player of the season - sealed Rotherham's fate.
The defeat left Rotherham, who hit the bar in the final minute through Joe Newell, four points adrift of Millwall with one match remaining.
Rotherham went down fighting having impressed against their high-flying opponents.
They made a bright start and midfielder Semi Ajayi came close to giving them the lead after 10 minutes, heading a cross from Anthony Forde over the bar.
The lively Ajayi then had a shot blocked two minutes later, before Rotherham were denied by the woodwork,
Newell swung over a corner in the 17th minute and Robertson rose above the Albion defence to meet the ball with a towering header that crashed against the post.
Robertson then connected with another Newell corner just after the half hour but this time his header flashed across the face of goal before dropping well wide.
Albion briefly came to life when Dwight Gayle had a cross-shot saved by Marek Rodak and then skipper Chris Brunt flashed a shot wide before they almost gifted Rotherham the lead at the end of the first half.
Central-defender Kyle Bartley tried to head clear a long throw but all he did was deflect the ball towards his own goal and he was relieved to see Craig Dawson clear off the line.
After their low-key display in the first half, Albion started the second period on the front foot and Stefan Johansen fired a shot just wide from Mason Holgate's 46th-minute cross.
But Rotherham's greater endeavour was rewarded when Newell floated a free-kick to the corner of the Albion area.
Robertson met the cross with a looping header that dropped over the line off the underside of the bar despite the best efforts of Brunt to hack the ball clear.
Ironically Albion enjoyed their best spell of the game after falling behind and drew themselves level when Gayle was fouled by Michael Smith and Rodriguez converted from the spot.
Their comeback was complete when Harper picked up a loose ball on the edge of the area and beat Rodak with a powerful low drive.